Image sensor with non-local readout circuit and optoelectronic device comprising said image sensor

ABSTRACT

Provided are image sensors with non-local readout circuits that include a substrate and a plurality of pixels and operatively connected to a control unit, wherein the control unit has first and second biasing circuits for providing, respectively, substantially symmetrical first and second biasing voltages and including, respectively, first and second selection means to selectively bias the pixels; and a readout circuit for reading out the pixels; and in that each pixel includes a photo-active element that has a photosensitizing layer associated to a transport layer; a non-photo-active reference element; first and second contacts circuitally connected, respectively, to the first and second biasing circuits; and an output contact circuitally connected to the readout circuit; wherein the photo-active element is circuitally connected between the first and output contacts, and the reference element is circuitally connected between the output and second contacts. Also provided are optoelectronic systems that include the image sensor.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of European Patent ApplicationEP15179484.9, filed Aug. 3, 2015, the disclosure of which isincorporated by reference herein in its entirety

TECHNICAL FIELD

The present invention relates to the field of image sensors, inparticular image sensors comprising a substrate, a plurality of pixelsarranged on a first area of the substrate, and a control unitoperatively connected to the plurality of pixels and adapted toselectively bias said pixels and read them out. An image sensoraccording to the present invention achieves an efficient integration ofthe plurality of pixels together with the control unit while avoidingin-pixel readout electronics, leading to more simple and compact pixels,and making the image sensor well-suited for integration in devices thatneed to be flexible and/or stretchable and/or transparent (or at leastpartially transparent) to the human eye. Moreover, the particular pixeldesign of the image sensors of the present invention makes it possibleto obtain pixels with high photoconductive gain, enhanced responsivityand/or improved sensitivity. The present invention also relates to anoptoelectronic device comprising said image sensor.

BACKGROUND

The use of image sensors is known in numerous applications ranging fromthe general-consumer gadgets sector, to the professional photography,and to industrial, medical and/or scientific uses, just to cite a few.

A typical image sensor comprises a plurality of pixels operativelyconnected to a control unit adapted to selectively bias said pixels andread them out. Each pixel includes a photo-active element orphotodetector, which is usually a photodiode.

The image sensor market is at present dominated by active pixel sensors(APSs), which are fully-compatible with the CMOS process. A typicalpixel in an APS comprises a photodiode for the collection of light, aswitching element (such as for example a transistor) to allow the pixelto be individually addressed during readout, and an amplifier.

Current technology trends in the APS design aim at the miniaturizationof the pixels while, at the same time, embedding more functionality inthe pixels to provide enhanced features, such as for instance globalshuttering or noise reduction among others. However, these conflictingtrends complicate the design of the pixel and that of the overall imagesensor.

As the size of the pixels shrinks, so does the size of theirphotodiodes. Given that the quantum efficiency of typical photodiodescannot exceed one for the visible and infrared ranges, APSs criticallyrely on reaching very low noise levels and/or on using long exposuretimes, to achieve high signal-to-noise ratios. Moreover, as more andmore transistors are required inside the pixel to implement suchadvanced functionality, the area available for light collection of thephotodiode (or pixel fill factor) is further decreased. Therefore, imagesensors with an improved pixel design and a more sophisticated readoutcircuit will be necessary to cope with the increasingly demandingperformance specifications.

Back-side illuminated image sensors have been developed in an attempt toovercome the reduction in pixel fill factor of conventional imagesensors (also referred to as front-side illuminated image sensors). In aback-side illuminated image sensor, the in-pixel readout electronics isarranged behind the semiconductor layer comprising the photodiode, asopposed to their front-side illuminated counterparts in which saidin-pixel readout electronics lays on the same semiconductor layer as thephotodiode or above. This is typically done by flipping thesemiconductor wafer during manufacturing and then thinning its reverseside so that the incoming light can impinge on the photodiode withoutpassing through the in-pixel readout electronics. Back-side illuminatedimage sensors achieve a substantial improvement in the pixel fill factorand, hence, in their photon-collecting ability, improvement which iseven more significant when the pixel-size is small. However, oneimportant shortcoming of back-side illuminated image sensors is thattheir manufacturing becomes dramatically more complicated and costly.

After APSs, the second largest portion of the market of image sensors isoccupied by charged-coupled devices (CCDs) which, although also using aphotodiode for light collection, their manufacturing and operation isquite different from that of APSs. In a CCD the charge generated by thecollection of photons at a given pixel, and initially stored in acapacitive storage element in said pixel, is then transferred fromwithin the device to a processing area where it can be converted to anelectrical signal. Typically, the transfer of the photo-collected chargeof the pixels to the processing area is done in a stepped andsynchronized manner in which the charge collected in a pixel of each row(or column) of a two-dimensional arrangement of pixels is progressivelyshifted by one row (or column) and stored in the capacitive storageelement of the pixel in the adjacent row (or column) until eventuallyreaching the processing area of the CCD.

Compared to APSs, CCDs do not require switching elements or amplifiersto be provided inside the pixel. However, one of the main drawbacks ofthis type of image sensors is that they need a more complex readoutelectronics to handle the charge shifting process. Moreover, CCDsrequire a dedicated manufacturing technology that is costly and, moreimportantly, incompatible with standard CMOS processing.

Another important aspect to take into account is the spectral range inwhich an image sensor is to operate as it will greatly determine thechoice of the available light-absorbing materials for the fabrication ofthe photo-active element of the pixels.

In that sense, silicon is widely used in image sensors operating in thevisible and near infrared ranges. In contrast, compounds such as InGaAsor HgCdTe, among others, are often employed for the infrared range(including short-wave infrared and/or long-wave infrared subranges).Finally, for image sensors operating in the ultraviolet region, andshorter-wave ranges, some known suitable materials include wide-gapsemiconductors, such as for instance AlGaN.

Image sensors that integrate silicon (e.g., CMOS technology) for theircontrol unit with photosensitive materials other than silicon for thephoto-active elements of the pixels (also referred to as hybrid imagesensors) offer an extended operating spectral range compared toCMOS-based image sensors. However, as for CMOS-based image sensors,hybrid image sensors do not provide a practical solution to thetechnological challenges of miniaturization and embedding morefunctionality at the pixel level, with the added disadvantage that suchhybrid integration involves difficult and costly bonding processes.

The rapid development in the recent years of a market for consumergadgets, wearable devices and mobile applications has stirred a growinginterest in the development of technology able to provide components,and even full devices, being flexible and/or stretchable and/ortransparent (or at least partially transparent) to the human eye.

Given that most of such devices incorporate image sensors, it would bedesirable to have an imaging technology able to provide flexible and/ortransparent image sensors. However, none of the imaging technologiesdescribed above is intended to produce image sensors with suchproperties.

Some image sensors have been proposed in an attempt to provide atransparent solution. For example, document U.S. Pat. No. 5,349,174 Adiscloses an image sensor having a two-dimensional arrangement of pixelsdisposed on a transparent substrate. In addition, the pixels of saidimage sensor comprise some elements, such as for instance a storingcapacitor, that are also transparent. Although the resulting imagesensor is semitransparent (as only a portion of the area occupied by thepixels is transparent), it is not intended to be flexible. Moreover, thecontrol unit of the image sensor requires in-pixel switching elementsfor addressing individual pixels upon readout, which reduces the pixelfill factor and increases the complexity of the pixel design and that ofthe readout circuit of the control unit.

There have also been some attempts to provide a flexible image sensor.For example, document U.S. Pat. No. 6,974,971 B2 describes an imagesensor that is bendable up to a certain extent, and that includes anarray of pixels disposed on discrete areas of a substrate. Selectedregions of the substrate, away from those areas in which the pixels areformed, are weakened to encourage flexing of the substrate to occurpreferentially at those regions upon bending of the device and, in thismanner, reduce the risk of damaging the pixels. Another example isdisclosed in U.S. Pat. No. 8,193,601 B2, in which an image sensorcomprises a plurality of pixels, each having a PIN photodiode asphoto-active element, disposed on a flexible substrate. However, thesesolutions are far from satisfactory as in-pixel selection elements, inparticular thin-film transistors (TFTs), are still required toselectively read out the pixels.

Photo-active elements based on organic photodiodes, as the onesdescribed in U.S. Pat. No. 6,300,612 B1, have also been thought of aspromising candidates for flexible and transparent image sensors.However, these image sensors will generally still need an in-pixelswitching element for addressing individual pixels. Moreover, organicphotodiodes have a fairly limited responsivity, well below 1 NW, whichmight be problematic when used in image sensors, especially in thosefeaturing small-sized pixels.

The use of active devices based on two-dimensional (2D) materials, suchas for instance graphene, for different applications is the object ofon-going research. For example, single-pixel photodetectors having aphotosensitive element made of graphene have been demonstrated as proofof concept. The use of photodetectors based on 2D materials (e.g.,graphene, as disclosed in for instance U.S. Pat. No. 8,053,782 B2) or onsemiconductor nanocrystals (e.g. quantum dots, see for example U.S. Pat.No. 8,803,128 B2) in the pixels of full-size image sensors has also beenproposed. However, such image sensors typically exhibit limitedphotoconductive gain.

Therefore, it would be highly desirable to have image sensors in whichthe photosensitive element of their pixels is capable of providing ahigh photoconductive gain, without compromising the pixel sensitivitydue to, for example, high dark current levels.

Document U.S. Patent Application Publication No. 2014/353471A1 describesa dark current suppression scheme based on a photosensitive and ashielded photodiode and which includes only one biasing circuit(providing bias voltage VRT, as shown in its FIG. 1). The schemeproposed in said document provides dark current compensation based ontemperature information and temperature dependent calibrationinformation.

Document PCT International Patent Application Publication No. WO2013/017605 A1 discloses a phototransistor comprising a transport layermade of graphene, and a sensitizing layer disposed above the transportlayer and that is made of colloidal quantum dots. The sensitizing layerabsorbs incident light and induces changes in the conductivity of thetransport layer to which it is associated. The high carrier mobility ofgraphene and the long carrier lifetime in the quantum dots make itpossible for the phototransistor disclosed therein to obtain a largephotoconductive gain. However, the device can only achieve desiredresponsivity levels at the expense of increased dark current levels,which in turn degrade the sensitivity and the shot-noise limit of thedevice.

Document U.S. Patent Application Publication No. 2014/0299741 A1 refersto a transparent ambient-light sensor using sensitized graphenephotodetectors that comprise two types of quantum dots arranged on asheet of graphene. By detecting the difference in response of the twotypes of quantum dots, the sensor can provide ambient light andbandwidth sensing. Although this solution works for a reduced numberphotodetectors, it is not scalable to imaging applications involving alarge number of pixels (typically a few millions), each pixel comprisinga photodetector, as the power consumption of the device to biassimultaneously all the pixels would be prohibitive for any practicalimage sensor. Moreover, the architecture of the ambient-light sensor isvery different from that of an image sensor, the latter requiring acontrol unit to selectively read out the pixels.

Paper “A CMOS image sensor with a double junction active pixel”, IEEETransactions on Electron Devices, Vol. 50, no. 1, pp 32-42, by FindlaterK. M. at al., discloses a CMOS image sensor that employs a verticallyintegrated double-junction photodiode structure. Some elements of theread-out circuit of the image sensor disclosed in said paper are local,i.e. are arranged at the pixel level. Specifically, for the arrangementshown in its FIG. 7, the pixels contain six active transistors to whichreset and read signal lines are connected, and which thereforeconstitute local elements of the read-out circuit. The photodiodesforming the image sensor disclosed in said paper cannot be considered asphotosensitizing elements, and the arrangement forming the image sensordoes not either comprise a transport layer for transporting electriccharge carriers.

It is therefore an object of the present invention to provide anenhanced image sensor in which the integration of its pixels with thecontrol unit can be done in a simple and efficient manner, whileavoiding a reduction in the pixel fill factor due to in-pixel read-outelectronics.

It is also an object of the present invention to provide an image sensorin which its pixels comprise an improved photo-active element capable ofhigh photoconductive gain, and/or enhanced responsivity.

It is a further object of the present invention to provide an imagesensor with an improved sensitivity of its pixels, and that does notrequire deep cooling of the device to achieve high signal-to-noiseratios.

It is yet another object of the present invention to provide an imagesensor well-suited for flexible and/or stretchable and/or transparentoptoelectronic devices.

SUMMARY

The objects of the present invention are solved with the image sensorwith non-local readout circuit of claim 1 and the optoelectronic deviceof claim 20. Other favorable embodiments of the invention are defined inthe dependent claims.

In the scope of the present invention the term image sensor refers to aphotodetector array of m×n pixels, where m and n can be any numberstarting at 1.

An aspect of the present invention relates to an image sensor withnon-local readout circuit comprising a substrate, a plurality of pixelsarranged on a first area of the substrate, and a control unitoperatively connected to the plurality of pixels and adapted toselectively bias said pixels and read them out. The image sensor ischaracterized in that the control unit comprises a first biasing circuitfor providing a first biasing voltage, a second biasing circuit forproviding a second biasing voltage, the second biasing voltage beingsubstantially symmetrical to the first biasing voltage with respect to avoltage reference, and a readout circuit for reading out thephoto-signal generated by the light impinging on the pixels.

The first biasing circuit and the second biasing circuit comprise,respectively, first selection means and second selection means toselectively bias one or more pixels of said plurality that are to beread out at a given time, the first selection means and the secondselection means being arranged outside the first area of the substrate.

In accordance with the present invention, the image sensor is furthercharacterized in that each pixel of the plurality of pixels comprises: aphoto-active element comprising a photosensitizing layer associated to atransport layer, the transport layer including at least one layer of atwo-dimensional material; a non-photo-active reference element disposedproximate to the photo-active active element, the reference elementhaving a dark conductance that substantially matches the darkconductance of the photo-active element; a first contact circuitallyconnected to the first biasing circuit; a second contact circuitallyconnected to the second biasing circuit; and an output contactcircuitally connected to the readout circuit.

Moreover, the photo-active element is circuitally connected between thefirst contact and the output contact, and the reference element iscircuitally connected between the output contact and the second contact.

The readout circuit is called non-local readout circuit because isarranged outside the first area of the substrate, and preferably all ofthe pixels of the above mentioned plurality of pixels are absent ofembedded readout electronics.

For a preferred embodiment, the first biasing circuit and the secondbiasing circuit are independent biasing circuits having their ownindependent control electronics providing the first biasing voltage andthe second biasing voltage, respectively.

The combination of a photo-active element with a non-photo-activereference element in the pixels of the image sensor makes it possible toobtain the full benefit of the high photoconductive gain and enhancedresponsivity of sensitized two-dimensional-material-based photodetectorswithout suffering the drawbacks of increased dark current levels, andits subsequent loss in pixel sensitivity.

The non-photo-active (or blind) reference element, together with theparticular interconnection of the photo-active element and the referenceelement, and their biasing with substantially symmetrical biasingvoltages, enable a balanced readout scheme of the photo-signal generatedin the photo-active element of the pixels that makes it possible tosubstantially suppress the dark current generated in the photo-activeelement of the pixel due to the biasing voltages during the exposurecycle.

In this way it is no longer needed to give up in terms of electricalperformance of the photo-active elements (e.g. in terms of responsivity)in order to keep the dark current levels low. In consequence, regardlessthe biasing voltages applied, the image sensor of the present inventionmakes it possible to obtain enhanced pixel sensitivity and highsignal-to-noise ratios, even without cooling the device.

The non-photo-active (or blind) reference element arranged in each pixelhas a dark conductance that substantially matches the dark conductanceof the photo-active element of the pixel to which said reference elementis associated. In this manner, the reference element simulates thebehavior of the photo-active element of said pixel during the exposurecycle.

In accordance with the present invention, the dark conductance of areference element of a pixel substantially matches the dark conductanceof the photo-active element of said pixel if the dark conductance of theformer does not differ from the dark conductance of the latter by morethan 25%, 20%, 15%, 15%, 10%, 8%, 3% or even 1%.

In some embodiments, the reference element of each pixel is individuallyfine-tuned so that its dark conductance closely matches the darkconductance of its associated photo-active element.

Moreover, because of the arrangement of the photo-active element betweenthe first contact and the output contact and the reference elementbetween the output contact and the second contact, when substantiallysymmetrical biasing voltages are applied to the first and secondcontacts of a given pixel, the voltage difference at the output contactof said pixel contains directly the photo-signal generated in said pixelby the incident light.

In case that the dark conductance of a reference element of a pixelexactly matched the dark conductance of the photo-active element of saidpixel, then the dark current generated in the photo-active element ofsaid pixel during the exposure cycle would be best suppressed by settingthe second biasing voltage to be exactly symmetrical to the firstbiasing voltages. However, in practical situations, a substantial matchbetween the dark conductance of the reference element of a pixel andthat of its associated photo-active element will be more likely than aperfect match. For that reason, it may be advantageous to set the firstand second biasing voltages to slightly different values, while stillbeing substantially symmetrical, in order to minimize the dark currentgenerated in the pixel. In other words, a slight amplitude “detune”between the first and second biasing voltages may efficiently compensatefor a residual mismatch between the dark conductance of the referenceand photo-active element of a pixel.

The photoconductive gain obtained from the photo-active element of thepixels advantageously eliminates the need for the pre-amplification ofthe photo-signal generated by the incident light inside the pixel,conversely to the pixels of APSs in which such pre-amplification isrequired.

In addition, the first and second selection means allow to selectivelybias the pixels of the image sensor enabling only the pixel or pixelsthat are to be read out at a given time, while leaving the other pixelsdisabled. In this way, the image sensor of the present invention doesnot require in-pixel selection elements for the readout process.

Given that the photo-active and reference elements can be directlyconnected between the first and second biasing contacts and the outputcontact without requiring any additional in-pixel electronics (such asamplifiers or selection elements), the pixel design is greatlysimplified, maximizing the area available for the collection of light.In this manner, it is possible to obtain smaller-sized pixels withoutcompromising the pixel fill factor, which can still be very high.

The high photoconductive gain of the photo-active element of the pixelscombined with the balanced biasing scheme of the pixels makes itpossible to transfer the readout electronics from inside the pixels tooutside the first area of the substrate occupied by the plurality ofpixels. The readout electronics can now be advantageously arranged onperipheral portions of said substrate or even on a different substrate,hence obtaining an image sensor with a non-local readout circuit.

In the context of the present invention, the term non-local readoutcircuit preferably refers to the fact that there is no readoutelectronics embedded in the pixels of the image sensor, incontraposition to the image sensors of the prior art, in which there isin-pixel readout electronics.

Finally, as no opaque and/or bulky electronics are required in the areaof the substrate occupied by the plurality of pixels, the resultingimage sensor is well-suited for integration into devices that need to beflexible and/or stretchable and/or transparent (or at least partiallytransparent) to the human eye.

According to the present invention, a device is considered to betransparent if at least the 80% of the incident light in the visiblepart of the spectrum is transmitted through said device. Similarly, adevice is considered to be partially transparent if at least 30% of theincident light in the visible part of the spectrum is transmittedthrough said device. Alternatively, a device is considered to be opaqueif less than 3% of the incident light in the visible part of thespectrum is transmitted through said device.

Also in accordance with present invention, a device being flexiblepreferably refers to a device that can be deformed, twisted, bent,rolled and/or folded (hence changing its shape or form) without beingdamaged or having its performance degraded.

Also in accordance with present invention, a device being stretchablepreferably refers to a device that can be deformed, strained, elongatedand/or widened (hence changing its shape or form) without being damagedor having its performance degraded.

In the context of the present invention the term two-dimensionalmaterial preferably refers to a material that comprises a plurality ofatoms or molecules arranged as a two-dimensional sheet with a thicknesssubstantially equal to the thickness of the atoms or molecules thatconstitute it.

In some embodiments, the transport layer of the photo-active element ofone or more pixels includes at least five, ten, twenty, forty or evenfifty layers of a two-dimensional material.

Also in the context of the present invention a photosensitizing layerbeing associated to a transport layer preferably refers to the fact thatlight absorption in the photosensitizing layer results in a change incharge carrier density inside the transport layer, which, for anembodiment, comprises graphene.

This can for example be due to the following processes:

An electron (or a hole) from an electron-hole pair generated in thephotosensitizing layer by the absorption of a photon can be transferredto the transport layer while the hole (or the electron) of saidelectron-hole pair remains trapped in the photosensitizing layer, or aninterface between the photosensitizing layer and the transport layer,such as for instance in a dielectric layer disposed there between. Insome embodiments, the photosensitizing layer is disposed above, such asfor example directly above, the transport layer. Alternatively, in someother embodiments the photosensitizing layer is disposed below, such asfor example directly below, the transport layer, so that a photon mustcross the transport layer before reaching the photosensitizing layerwhere it will be absorbed.

Alternatively, light absorption in the photosensitive layer leads tobound charges in the proximity of the surface of the photosensitivelayer. This draws charges into the graphene and/or into any othermaterial forming the transport layer, which changes its electricalconductivity.

In this sense, the heterojunction formed by the photosensitizing layerand the transport layer slows down recombination and makes it possibleto collect several electric carriers for a single absorbed photon, whichcompounded with the high carrier mobility of the two-dimensionalmaterial comprised in the transport layer, results in the photo-activeelement of the pixels featuring very high photoconductive gain andresponsivity.

In addition, the spectral sensitivity of the photo-active element of thepixels can be advantageously tailored by appropriately selecting thematerial of the photosensitizing layer. In this manner, the spectralrange for photodetection of the photo-active element can be extendedover a large bandwidth.

In some embodiments, the photosensitizing layer of the photo-activeelement of one or more pixels comprises a photo-absorbing semiconductor,a 2D material, a polymer, a dye, quantum dots (such as for instancecolloidal quantum dots), a ferroelectric material, Perovskite and/or acombination thereof.

The photosensitizing layer may for example comprise nanocomposite filmscontaining blends of the aforementioned materials. It may also be asingle-layered structure or, alternatively, a multi-layered structure,in which one or more of the aforementioned materials constitutedifferent layers stacked on each other, each having thicknessespreferably between approximately 5 nm and approximately 400 nm.

In those embodiments in which the photosensitizing layer comprisesquantum dots, these are preferably of one or more of the followingtypes: Ag₂S, Bi₂S₃, CdS, CdSe, CdHgTe, Cu₂S, CIS (copper indiumdisulfide), CIGS (copper indium gallium selenide), CZTS (copper zinc tinsulfide), Ge, HgTe, InAs, InSb, ITO (indium tin oxide), PbS, PbSe, Si,SnO₂, ZnO, and ZnS.

Similarly, in some embodiments the at least one layer of atwo-dimensional material comprised in the transport layer of thephoto-active element of one or more pixels comprises one or more of thefollowing materials: graphene, MoS₂, MoSe₂, WS₂, WSe₂, black phosphorus,SnS₂, and h-BN (hexagonal boron nitride).

In the context of the present invention, two voltages are considered tobe substantially symmetrical (in particular substantially symmetricalwith respect to a voltage reference) if they have opposite signs withrespect said voltage reference and the magnitude of one differs from themagnitude of the other in less than a 25%, 20%, 15%, 10%, 8%, 5%, 3% oreven 1%.

Also in the context of the present invention, a layer (or an element, ora contact, or a device) of the image sensor is considered to be aboveanother, if the former is farther from the substrate of the image sensorthan the latter, along a direction perpendicular to said substrate.

Similarly, a layer (or an element, or a contact, or a device) of theimage sensor is considered to be below another, if the former is closerto the substrate of the image sensor than the latter, along saidperpendicular direction.

Also in accordance with the present invention, the term above (or below)is not to be construed as implying than one layer (or an element, or acontact, or a device) is immediately or directly above (or below)another unless explicitly stated otherwise. In that sense, a layer beingdisposed above (or below) another does not preclude the possibility ofadditional layers being arranged in between those two.

In the same manner, in the context of the present invention the termcircuitally connected preferably refers to the fact that a first entity(e.g., a contact, an element or a circuit) may be connected to a secondentity by means of a circuit, which may comprise one or more conductivetraces and/or one or more circuit components operatively arrangedbetween said two entities. Thus, the term circuitally connected is notto be construed as requiring a direct ohmic connection of the firstentity to the second entity (i.e., without any intervening circuitcomponents) unless explicitly stated.

In some embodiments, the first selection means and/or the secondselection means advantageously comprise a plurality of switches or amultiplexer.

In some embodiments the first contact and the output contact of at agiven pixel are disposed above the transport layer of the photo-activeelement of said pixel, whereas in other embodiments said first contactand output contact are disposed below the transport layer of saidphoto-active element. In yet other examples, one of said two contacts isdisposed above the transport layer of the photo-active element of thepixel while the other is disposed below the transport layer of thephoto-active element.

In certain cases, the first, second and/or output contact of one or morepixels of the plurality of pixels are made of a transparent conductingoxide, such as indium tin oxide (ITO).

In some examples the control unit is disposed on a second area of thesubstrate, said second area not overlapping said first area on which theplurality of pixels are arranged. However, in other examples, thecontrol unit is disposed on another substrate provided in the imagesensor.

In a first group of embodiments, the reference element of at least onepixel of the plurality of pixels comprises a transport layer, saidtransport layer including at least one layer of a two-dimensionalmaterial. Preferably, said reference element further comprises aphotosensitizing layer associated to the transport layer of thereference element.

As the structure of the reference element mimics that of thephoto-active element of the pixel, it is possible to obtain in a simplemanner a reference element with a dark conductance that accuratelymatches the dark conductance of the photo-active element.

In these embodiments, the second contact and the output contact of at agiven pixel may be disposed both above, both below, or one above and theother below the transport layer of the reference element of said pixel.

In some examples in which the reference element of said at least onepixel comprises a transport layer and a photosensitizing layerassociated thereto, said reference element further comprise a firstlight-blocking layer disposed above the photosensitizing layer and thetransport layer of said reference element.

The first light-blocking layer advantageously covers thephotosensitizing layer and the transport layer of said referenceelement, ensuring that no photo-signal is generated in the referenceelement by the light impinging on the image sensor. Otherwise, theconductance of said reference element would be undesirably modified and,hence, its ability to subtract the dark current component from thephoto-signal generated at the photo-active element of the pixel would bedegraded.

More preferably, the reference element of said at least one pixel alsocomprises a second light-blocking layer disposed below thephotosensitizing layer and the transport layer of said referenceelement.

The second light-blocking layer protects the photosensitizing layer andthe transport layer of said reference element from light that couldarrive through the substrate of the image sensor, as it could happen inthose cases in which the image sensor comprises a thin and/ortransparent substrate.

In the context of the present invention the term light-blocking layerpreferably refers to the fact that said layer is opaque for the range ofwavelengths of operation of the photo-active element of the plurality ofpixels. However, said layer may at the same time be transparent, or atleast partially transparent, to the human eye.

Alternatively, the image sensor may comprise a substrate that is opaquefor the range of wavelengths of operation of the photo-active element ofthe plurality of pixels. Such feature advantageously eliminates the needfor a second light-blocking layer in the reference element of said atleast one pixel.

In an embodiment, the first and/or second light-blocking layers take theform of a passivation layer, said passivation layer preferablycomprising an oxide.

Alternatively, in other instances of such cases, the photosensitizinglayer of the reference element of said at least one pixel is notsensitive in the range of wavelengths of operation of the photo-activeelement of the said pixel.

This results in a simpler reference element design because it eliminatesthe need for light-blocking layers, as the light impinging on saidreference element cannot be absorbed by its photosensitive layer.

In the context of the present invention, a photosensitizing layer of thereference element of a pixel is considered not to be sensitive in therange of wavelengths of operation of the photo-active element of saidpixel if the spectral absorbance of the photosensitizing layer of saidreference element at any given wavelength within that range is smallerthan a 25% of the lowest spectral absorbance of the photo-active elementfor the range of wavelengths of operation.

In some embodiments of this first group, the transport layer of thereference element of said at least one pixel has a smaller area than thetransport layer of the photo-active element. In this way, the overheadin real estate due to the presence of the reference element in the pixelis minimized. In order to avoid altering the dark conductance of thereference element, which must substantially match the dark conductanceof the photo-active element contained in the same pixel, the transportlayer of the reference element may preferably have the same shape (orgeometry or form factor) as the transport layer of the photo-activeelement.

Alternatively, in case that the transport layer of the reference elementand that of the photo-active element of a pixel have different shapes,then the doping of the transport layer of the reference element can beadvantageously varied with respect to the doping of the transport layerof the photo-active element so that the dark conductance of the formersubstantially matches the dark conductance of the latter.

In some cases, the transversal dimensions of the reference element ofone or more pixels of the plurality of pixels are below the diffractionlimit for the range of wavelengths of operation of the photo-activeelement of said pixels. In this way, the reference element of saidpixels does not block any light incident on the image sensor.

Optionally, the reference element of at least one pixel of the pluralityof pixels is arranged between the substrate and the photo-active elementof said pixel. Such an arrangement advantageously exploits the thirddimension of the structure to obtain a more compact architecture.Moreover, by disposing the reference element below the photo-activeelement, light absorption by the transport layer and/or thephotosensitizing layer of the reference element is further prevented.

However, in other embodiments the reference element of a pixel isdisposed on a same level as the photo-active element of said pixel. Insome examples, the image sensor further comprises one or more primaryinsulating layers associated to the photo-active element of theplurality of pixels. In these examples, at least one pixel of theplurality of pixels preferably comprises:

-   -   a back-gate contact disposed between the substrate and the        photo-active element of said at least one pixel, between a        primary insulating layer and the substrate, wherein said primary        insulating layer is disposed between said photo-active element        and the substrate; and/or    -   a top-gate contact disposed above the photo-active element of        said at least one pixel.

By providing a back-gate contact and/or a top-gate contact, thephoto-active element of the pixels can be gated to finely control theconduction and photosensitivity of the photosensitizing layer.

Preferably, the top-gate contact and/or the back-gate contact is made ofa transparent material, so as to not hinder the light absorptioncapabilities of the photo-active element of the pixels.

In those cases in which a pixel comprises a top-gate contact disposedabove its photo-active element, the image sensor preferably comprises a(or a further) primary insulating layer disposed between said top-gatecontact and the photo-active element of said pixel.

In some embodiments of said first group, the image sensor may alsocomprise one or more secondary insulating layers associated to thereference element of the plurality of pixels. Then, in such embodimentsat least one pixel of the plurality of pixels preferably comprises:

-   -   a back-gate contact disposed between the substrate and the        reference element of said at least one pixel, between a        secondary insulating layer and the substrate, wherein said        secondary insulating layer is disposed between said reference        element and the substrate; and/or    -   a top-gate contact disposed above the reference element of said        at least one pixel.

By providing a back-gate contact and/or a top-gate contact, thereference element of the pixels can be gated to finely control itsconductance.

Moreover, in those cases in which a pixel comprises a top-gate contactdisposed above its reference element, the image sensor preferablycomprises a (or a further) secondary insulating layer disposed betweensaid top-gate contact and the reference element of said pixel.

In accordance with the present invention, a primary insulating layerassociated to a photo-active element preferably refers to the fact thatsaid insulating layer is disposed above (such as for instance directlyabove) or alternatively below (such as for instance directly below) boththe transport layer and the photosensitizing layer of said photo-activeelement.

Similarly, also in accordance with the present invention, a secondaryinsulating layer associated to a reference element preferably refers tothe fact that said insulating layer is disposed above (such as forinstance directly above) or alternatively below (such as for instancedirectly below) said reference element. In that sense, if a referenceelement comprises a transport layer and a photosensitizing layer, thenthe secondary insulating layer would be above or below both layers ofsaid reference element.

Preferably, said one or more primary and/or secondary insulating layerscomprise an oxide.

In some cases, the image sensor further comprises an encapsulation layerdisposed above the plurality of pixels. In this manner, the photo-activeelements and the reference elements of the pixels are advantageouslyprotected. Preferably, the encapsulation layer comprises a dielectricmaterial having a wide bandgap, to minimize the absorption of light atthe wavelengths of operation of the photo-active elements.

In some embodiments of the image sensor of the present invention, theplurality of pixels are grouped into clusters, each cluster comprisingone or more pixels, with the photosensitizing layer of the photo-activeelement of the one or more pixels of each cluster being sensitive to adifferent range of the spectrum.

This makes it possible to obtain an image sensor with an extendedfrequency range of operation, covering from X-ray photons and theultraviolet (UV) to the infrared (IR), including near-infrared (NIR),short-wave infrared (SWIR), mid-wave infrared (MWIR) and long-waveinfrared (LWIR), and even THz frequencies. It also allows implementingimage sensors having multicolor pixels by, for example, tailoring theproperties of the material selected for the photosensitizing layer.

The image sensor and the optoelectronic system of the present inventioncan also be applied to spectrometry, thus constituting a spectrometer.

In a preferred embodiment of the image sensor of the present invention,the plurality of pixels are arranged as a two-dimensional arraycomprising a plurality of rows, each row comprising the same number ofpixels. In said embodiments, the first selection means and the secondselection means comprise, respectively, first row-select switches andsecond row-select switches to selectively bias the rows of the array.

The first and second row-select switches make it possible to enable onlyone row (or a few rows) of the array while leaving the other rowsdisabled. In this manner, the power consumption of the image sensorduring operation is advantageously reduced.

Preferably, the control unit is operatively connected to the firstrow-select switches and the second row-select switches, and isconfigured to sequentially read out the rows of pixels by activating thefirst row-select switch and the second row-select switch of one row at atime.

By biasing the rows sequentially, the connection of the pixels of thearray to the readout circuit is greatly simplified, as pixels located indifferent rows (e.g. the pixels forming a column in the two-dimensionalarray) can be, for instance, daisy-chained to the readout circuit. Insuch configuration, at any time during the readout process, the pixelsin the non-selected rows remain disabled without loading the electricalpath that connects a given pixel of the selected row with the readoutcircuit.

In some examples of said preferred embodiment, the readout circuitcomprises:

-   -   a multiplexer comprising as many input terminals as there are        pixels in each row and an output terminal, each input terminal        of the multiplexer being circuitally connected to the output        contact of a pixel of each row; and    -   an amplifier operatively connected in series to the output        terminal of the multiplexer.

In addition, in said examples the readout circuit optionally comprises astorage element configured to store a voltage proportional to thephoto-signal generated in a pixel of the plurality of pixels, thestorage element being operatively connected in series to the amplifier.

Given that most of the readout electronics is shared by all the pixelsof the two-dimensional array, in these examples the overhead in realestate due to the readout circuit is minimized.

Alternatively, in some other examples of said preferred embodiment, thereadout circuit comprises:

-   -   as many amplifiers as there are pixels in each row, each        amplifier having an input terminal, circuitally connected to the        output contact of a pixel of each row, and an output terminal;        and preferably    -   a storage element connected in series to the output terminal of        each amplifier, each storage element being configured to store a        voltage proportional to the photo-signal generated in a pixel of        the plurality of pixels.

Such a case constitutes a good design trade-off, as the additional realestate requirements to accommodate a different amplifier for the pixelsforming each column of the array is counterbalanced with a faster pixelreadout and more robustness to noise, without increasing the complexityof the pixel design.

In some further examples of said preferred embodiment the readoutcircuit comprises:

-   -   for each column of a first group of columns of the        two-dimensional array, a single amplifier circuitally connected        to the output contact of the pixels of said column; and    -   for the columns of a second group of columns of the        two-dimensional array, an amplifier circuitally connected to the        output contact of the pixels of the columns of said second        group.

This option advantageously provides greater flexibility to tailor theprocessing of the photo-signals generated in different areas of theimage sensor.

Yet in some other examples of said preferred embodiment, at least onepixel of the plurality of pixels comprises an amplifier embedded insidethe pixel. Preferably, said at least one pixel also comprises a storageelement connected in series to an output terminal of said amplifier.

In-pixel amplification makes the pixel more robust to noise and allowsfaster pixel readout, improving the scalability of the pixel array ofthe image sensor, which may be preferred for those applications of theimage sensor in which high bandwidth and throughput is required.

The control unit preferably includes an interconnection circuit (such asfor example, but not limited to, a multiplexer) operatively connected tothe readout circuit and that comprises one or more output nodes. Theinterconnection circuit allows circuitally connecting, through thereadout circuit, the output contact of any of the pixels of the arraywith at least one of the one or more output nodes.

In some embodiments, the control unit comprises a post-amplificationstage operatively connected to at least one output node of the one ormore output nodes of the interconnection circuit.

Optionally, the control unit further comprises a correlation doublesampling stage operatively connected between said at least one outputnode of the interconnection circuit and the post-amplification stage.The correlation double sampling stage advantageously removes anyundesired offset in the values detected from the photo-signals read outfrom the pixels and reduces readout noise components.

Also optionally, the control unit further comprises an analog-to-digitalconverter operatively connected after the post-amplification stage. Inthis way, the image sensor outputs can be directly interfaced withdigital circuitry, such as for example a field-programmable gate array(FPGA), a digital signal processor (DSP), a microprocessor or amicrocontroller.

In certain embodiments of the image sensor of the present invention, thesubstrate is of a flexible and/or stretchable and, preferably,transparent material. The substrate may be made of polyethyleneterephthalate (PET) or polyethylene naphthalate (PEN) among otherpossible materials.

In this way, the mechanical and/or optical properties of the substratenicely match those of the materials used in the photosensitizing layerand/or the transport layer of the photo-active elements or the referenceelements of the pixels, making it possible to obtain a truly flexibleand/or stretchable and/or transparent image sensor.

Optionally in said embodiments, the image sensor further comprisesconductive traces that connect the first biasing circuit, the secondbiasing circuit and the readout circuit with, respectively, the first,second and output contacts of the pixels of the plurality of pixels. Inaddition, said conductive traces are made of a flexible and/orstretchable and/or transparent conductive material.

Said conductive traces run across said first area of the substrate andfrom/to the control unit located on peripheral portions of thesubstrate, outside said first area, and connect the first and secondcontacts of the pixels with, respectively, the first and second biasingcircuits and the output contact of the pixels with the readout circuit.

In some examples, at least some of said conductive traces are made of atransparent conducting oxide, such as indium tin oxide (ITO), althoughin other examples they can be made of other metallic (and generallyconductive) materials as long as they have flexible and/or stretchableand/or transparent properties.

Additionally, when said conductive traces are made of a flexible and/orstretchable material that is not transparent, said conductive traces canbe thinned sufficiently so as to have a width below the diffractionlimit for the range of wavelengths of operation of the photo-activeelement of said pixels.

Another aspect of the present invention relates to an optoelectronicdevice that comprises an image sensor according to the presentinvention.

In some embodiments, the optoelectronic device is a wearable device,such as for example but not limited to a wristwatch, a device adapted tobe attached to the body, a piece of clothing (e.g., textile), abracelet, eyeglasses or goggles. A flexible and/or stretchable imagesensor according to the present invention can be advantageously affixedto, or embedded in, a wearable device.

In some alternative or complementary embodiments, the optoelectronicdevice comprises a transparent panel, such as a windshield, a window, ora screen of a portable device (e.g., a smartphone or a tablet), on whichthe image sensor is disposed. Preferably, said transparent panel is madeof glass, plastic, or a flexible and/or stretchable material.

The transparency and flexibility properties than can be obtained withthe image sensors according to the present invention make these imagesensors well suited for consumer gadgets in general, portable devicesand/or mobile applications among others. However, these image sensorscan also be advantageously integrated into medical devices or devicesfor automotive applications, among others.

A method for manufacturing an image sensor with a non-local readoutcircuit such as described above, in which the image sensor comprises aplurality of pixels operatively connected to a control unit adapted toselectively bias said pixels and read them out, comprises the steps of:

-   -   a) providing a transport layer including at least one layer of a        two-dimensional material, and a photosensitizing layer        associated to the transport layer, on a first area of a        substrate;    -   b) providing a first biasing circuit, a second biasing circuit        and a readout circuit in the control unit, the first biasing        circuit providing a first biasing voltage, the second biasing        circuit providing a second biasing voltage substantially        symmetrical to the first biasing voltage, and the readout        circuit being adapted to read out the photo-signal generated by        the light impinging on the pixels;    -   c) arranging first selection means and second selection means        provided, respectively, in the first biasing circuit and the        second biasing circuit outside the first area of the substrate,        the first selection means and second selection means being        adapted to selectively bias one or more pixels of said plurality        that are to be read out at a given time;        -   wherein, for each pixel of the plurality of pixels, the            method further comprises:    -   d) defining a photo-active element at a selected location of the        transport layer and the photosensitizing layer arranged on the        first area of the substrate, and circuitally connecting the        photo-active element between a first contact and an output        contact provided in said pixel;    -   e) arranging a non-photo-active reference element proximate to        the photo-active active element of said pixel, the reference        element having a dark conductance that substantially matches the        dark conductance of the photo-active element, and circuitally        connecting the reference element between said output contact and        a second contact provided in said pixel;    -   f) circuitally connecting the first contact, the second contact,        and the output contact of said pixel to, respectively, the first        biasing circuit, the second biasing circuit, and the readout        circuit of the control unit.

BRIEF DESCRIPTION OF THE FIGURES

In the following some preferred embodiments of the invention will bedescribed with reference to the enclosed Figures. They are provided onlyfor illustration purposes without however limiting the scope of theinvention.

FIG. 1 is a schematic block diagram of an exemplary image sensoraccording to the present invention.

FIGS. 2A and 2B correspond to a bottom plan view and a cross-sectionalview of a pixel for the image sensor of FIG. 1, in which the first,second and output contacts of the pixel are disposed below the transportlayer of the photo-active element and the transport layer of thereference element of the pixel.

FIGS. 3A and 3B show, in a bottom plan view and a cross-sectional view,an alternative pixel layout for the image sensor of FIG. 1, in which thefirst, second and output contacts of the pixel are disposed above thetransport layer of the photo-active element and the transport layer ofthe reference element of the pixel.

FIG. 4 depicts a cross-sectional view of a pixel for an image sensoraccording to the present invention, in which the transport layer of thereference element of the pixel has a smaller area than the transportlayer of the photo-active element of said pixel.

FIG. 5 corresponds to a cross-sectional view of a pixel suitable for animage sensor in accordance with the present invention, in which thereference element of the pixel is arranged below the photo-activeelement of said pixel.

FIGS. 6A and 6B are a bottom plan view and a cross-sectional view of apixel for an image sensor according to the present invention, in whichthe pixel comprises a back-gate contact below the photo-active element.

FIGS. 7A and 7B depict a bottom plan view and a cross-sectional view ofanother pixel for an image sensor according to the present invention, inwhich the pixel comprises a back-gate contact below each of thephoto-active element and the reference element.

FIG. 8A shows a schematic block diagram of an embodiment of an imagesensor according to the present invention in which the readout circuitcomprises a multiplexer followed by an amplifier and a storage elementcascaded thereto.

FIG. 8B is a schematic block diagram of another embodiment of an imagesensor according to the present invention in which the readout circuitcomprises as many amplifiers as there are pixels in each row of thearray of pixels and a storage element connected in series to the outputnode of each amplifier.

FIG. 9 is a detailed representation, in a cross-sectional view, of areaA in FIG. 1 in which it is illustrated the crossing of differentconductive traces.

FIGS. 10A-10G depict the different steps in the process of fabricationof a pixel of the image sensor of FIG. 1.

FIG. 11 is a schematic representation of an exemplary image sensor inwhich its pixels are grouped into clusters, each cluster being sensitiveto a different range of the spectrum.

FIG. 12 shows a block diagram of an optoelectronic device in accordancewith an embodiment of the present invention.

FIGS. 13A and 13B are, respectively, a side view and a plan view of theimage sensor of the present invention for an embodiment for which alight-concentrating structure is arranged on top of the image sensor.

FIGS. 14A and 14B are, respectively, a side view and a plan view of theimage sensor of the present invention for an embodiment for which amicro-lens is arranged on top of each pixel.

FIG. 15 is a plot showing the normalized spectral response of threedifferent pixels of the image sensor of the present invention for anembodiment for which the image sensor is capable of multispectralresponse by the inclusion therein of pixels with photosensitizing layerssensitive to different ranges of the light spectrum, comprising quantumdots (QD) having different sizes (per pixel), where the curves relate toShort-wave Infrared (SWIR), Near Infrared (NIR) and visible light (VIS).

FIG. 16 shows several curves representative of data obtained from apixelated detector built according to the image sensor of the presentinvention where the light is transmitted through a diffractive opticssystem before it hits the pixelated detector. Each curve corresponds todata obtained when the combined system (diffractive optics coupled tothe pixelated detector according to the present invention) isilluminated with light of a specific wavelength (corresponding to thewavelength where the maximum in each curve occurs).

FIG. 17 shows several curves representative of data extracted from a4-pixel photodetector linear array according to the image sensor of thepresent invention, arranged on a flexible and transparent substrate.

DETAILED DESCRIPTION

In FIG. 1 it is illustrated a schematic block diagram of an embodimentof an image sensor with non-local readout circuit according to presentinvention. The image sensor 100 comprises a plurality of pixels 101arranged as a two-dimensional array of M rows and N columns on a firstarea 102 a of a substrate 102. In particular, FIG. 1 corresponds to abottom plan view of the image sensor 100, that is, as seen through thesubstrate 102.

The image sensor 100 further comprises a control unit operativelyconnected to the plurality of pixels 101 and adapted to selectively biassaid pixels and read them out. The control unit comprises a firstbiasing circuit 103 a for providing a first biasing voltage V_(DD), asecond biasing circuit 103 b for providing a second biasing voltageV_(SS), the second biasing voltage V_(SS) being substantiallysymmetrical to the first biasing voltage V_(DD), and a readout circuit104 for reading out the photo-signal generated by the light impinging onthe pixels 101. The control unit also includes a plurality of outputnodes 111 operatively connected to the readout circuit 104.

The first biasing circuit 103 a and the second biasing circuit 103 bcomprise, respectively, first selection means 105 a and second selectionmeans 105 b to selectively bias one or more pixels 101 of said pluralitythat are to be read out at a given time. The first selection means 105 aand the second selection means 105 b are arranged outside the first area102 a of the substrate 102 and, as illustrated in the example of FIG. 1,comprise a plurality of switches (implemented as gate-controlledtransistors).

Each pixel 101 of the plurality of pixels comprises a photo-activeelement 106 and a non-photo-active reference element 107 disposedproximate to the photo-active active element 106. Moreover, each pixel101 further comprises a first contact 108 a circuitally connected to thefirst biasing circuit 103 a, a second contact 108 b circuitallyconnected to the second biasing circuit 103 b, and an output contact 109circuitally connected to the readout circuit 104.

The photo-active element 106 is circuitally connected between the firstcontact 108 a and the output contact 109, while the reference element107 is circuitally connected between the output contact 109 and thesecond contact 108 b. The reference element 107 has a dark conductancethat substantially matches the dark conductance of the photo-activeelement 106, making it possible to substantially suppress the darkcurrent generated in the photo-active element 106 during the exposurecycle.

As it can be seen in greater detail in the cross-sectional view of FIG.2B, the photo-active element 106 comprises a photosensitizing layer 201associated to a transport layer 202 that includes at least one layer ofa two-dimensional material. Similarly, the reference element 107 alsocomprises a photosensitizing layer 203 associated to a transport layer204 that includes at least one layer of a two-dimensional material.

In this example the photosensitizing layer 201 of the photo-activeelement 106 and the photosensitizing layer 203 of the reference element107 are disposed above (and, in particular, directly above) thetransport layer 202 and 204 respectively. However, in other examples thephotosensitizing layer of the photo-active element or that of thereference element can be disposed below its corresponding transportlayer.

The image sensor 100 further comprises first conductive traces 110 a andsecond conductive traces 110 b that connect the first biasing circuit103 a and the second biasing circuit 103 b with, respectively, the firstcontact 108 a and the second contact 108 b of the pixels. In the exampleof the FIG. 1, said first and second conductive traces 110 a, 110 bextend horizontally across the first area 102 a of the substrate fromthe first and second biasing circuits 103 a, 103 b located on theleftmost and rightmost portions of the substrate 102, outside the firstarea 102 a.

Additionally, the image sensor 100 also comprises third conductivetraces 110 c (which in FIG. 1 extend along the vertical direction) thatconnect the output contacts 109 of the pixels in a daisy-chainconfiguration with the readout circuit 104, which is arranged in theuppermost portion of the substrate 102, outside said first area 102 a.

The substrate 102 is made of a flexible and transparent material, suchas for example PET or PEN. In addition, the first contact 108 a, thesecond contact 108 b and the output contact 109 of the pixels 101, andsaid conductive traces 110 a, 110 b, 110 c, are made of a transparentconducting oxide, such as for instance ITO.

In the image sensor 100, the first biasing circuit 103 a, the second 103b, and the readout circuit 104 (the three of them being comprised in thecontrol unit of the image sensor 100) are arranged on a second area 102b located on the periphery of the same substrate 102, hence notoverlapping the first area 102 a on which the plurality of pixels 101are arranged. However, in other examples, the control unit may bearranged on different substrate provided in the image sensor.

Referring now to FIGS. 2A and 2B, it is shown the layout of a pixel 101of the image sensor 100 in which the photo-active element 106 isarranged next to the reference element 107 on a same level. In thisexample, the transport layer 202 of the photo-active element 106 and thetransport layer 204 of the reference element 107 are coplanar. The firstcontact 108 a and the output contact 109 (at opposite ends of thephoto-active element 106) are disposed below the transport layer 202,while the second contact 108 b and the output contact 109 (at oppositeends of the reference element 107) are disposed below transport layer204.

The reference element 107 further comprises a first light-blocking layer205 disposed above the photosensitizing layer 203 and the transportlayer 204, and a second light-blocking layer 206 disposed below saidphotosensitizing layer 203 and said transport layer 204. In particular,the first light-blocking layer 205 is disposed directly above thephotosensitizing layer 203, while the second light-blocking layer 206 isseparated from the transport layer 204 by an insulating layer 207. Thefirst and second light-blocking layers 205, 206 are passivation layerscomprising an oxide.

FIGS. 3A and 3B depict an alternative example of a pixel layout than canbe used in the image sensor 100 of FIG. 1. For simplicity, elements incommon with the pixel structure of FIGS. 2A and 2B have been labeledwith the same reference numerals. The photo-active element 106 iscircuitally connected between a first contact 308 a and an outputcontact 309, while the reference element 107 is circuitally connectedbetween the output contact 309 and a second contact 308 b. Conversely tothe case illustrated in FIGS. 2A and 2B, now the first contact 308 a andthe output contact 309 are disposed above the transport layer 202, morespecifically between said transport layer 202 and the photosensitizinglayer 201. In the same way, the second contact 308 b and the outputcontact 309 are disposed above the transport layer 204, between saidtransport layer 204 and the photosensitizing layer 203.

The transport layers 202, 204 are spaced from the substrate 102 by meansof an insulating layer 307, which provides mechanical support for thedeposition of the output contact 309 in the region between saidtransport layers 202, 204.

Referring back to FIG. 1, it can be observed that the first and secondconductive traces 110 a, 110 b cross at a number of places the thirdconductive traces 110 c. To avoid the electrical contact betweendifferent conductive traces, the third conductive traces 110 c areraised to pass above the first and second conductive traces 110 a, 110b. An intervening insulating layer further prevents the electricalcontact between traces. For this reason, the third conductive traces 110c advantageously comprise a vertical portion (such as a via) through theintervening insulating layer to make ohmic connection with the outputcontact 109 of the pixels.

One of such crossings, in particular the one occurring in region A ofthe image sensor of FIG. 1, is illustrated in the cross-sectional viewof FIG. 9, in which a third conductive trace 110 c crosses above asecond conductive trace 110 b, both traces being spaced by anintervening insulating layer 900. The third conductive trace 110 ccomprises a vertical portion 901 that goes through the interveninginsulating layer 900 to reach the level on which the output contact 109of the pixels are arranged.

Alternatively, the pixels of the image sensor may advantageously havethe first and second contacts 108 a, 108 b disposed below the transportlayers 202, 204, and the output contact 109 disposed above the transportlayers 202, 204. In this case, as the first and second conductive traces110 a, 110 b will always be below the third conductive traces 110 c,electrical contact between traces is avoided. Moreover, the thirdconductive traces 110 c might no longer require vertical portions tomake ohmic connection with the output contact 109 of the pixels.Nevertheless, even in this case, it is still preferred to have anintervening insulating layer to further isolate the first and secondconductive traces from the third conductive traces.

Referring now to FIG. 4, it is there shown in a cross-sectional viewanother example of a pixel suitable for an image sensor in accordancewith the present invention. In particular, a pixel 401 is arranged on asubstrate 400 and comprises photo-active element 402 and a referenceelement 403 disposed one next to the other in a coplanar configuration.The photo-active element 402 is circuitally connected between a firstcontact 410 a and an output contact 409, while the reference element 403is circuitally connected between the output contact 409 and a secondcontact 410 b. Moreover, an insulating layer 413 has been provided onthe substrate 400, below photo-active element 402 and a referenceelement 403.

The photo-active element 402 comprises a photosensitizing layer 405associated to a transport layer 406, which is disposed below thephotosensitizing layer 405 and includes at least one layer of atwo-dimensional material. Likewise, the reference element 403 alsocomprises a photosensitizing layer 407 associated to another transportlayer 408, which is disposed below the photosensitizing layer 407 andincludes at least one layer of a two-dimensional material. The firstcontact 410 a, second contact 410 b, and output contact 409 aresandwiched between the photosensitizing layers 405, 407 and thetransport layers 406, 408.

In this example, the transport layer 408 of the reference element has asmaller area than the transport layer 406 of the photo-active element,advantageously reducing the overhead in real estate due to the presenceof the reference element 403 in the pixel 401. Despite being smaller insize, the transport layer 408 has the same shape as the transport layer406 in order to ensure that the dark conductance of the referenceelement 403 substantially matches the dark conductance of thephoto-active element 402.

Finally, as in the previous examples, the reference element 403 alsocomprises a first light-blocking layer 411 disposed above thephotosensitizing layer 407 and a second light-blocking layer 412disposed below the transport layer 408, so that the absorption of theincident light in the reference element 403 is prevented.

A further example of a pixel suitable for an image sensor according tothe invention is depicted in FIG. 5, in which a pixel 501 is disposed ona substrate 500 and comprises a reference element 503 arranged below aphoto-active element 502, resulting in very compact architecture withreduced footprint.

The photo-active element 502 comprises a photosensitizing layer 504disposed above a transport layer 505. Below the photo-active element502, the reference element 503 also comprises a photosensitizing layer506 disposed above another transport layer 507. A primary insulatinglayer 512 associated to the photo-active element 502 is arranged betweenthe photo-active element 502 and the reference element 503, to provideisolation between the two elements.

The reference element 503 comprises a first light-blocking layer 511disposed above the its photosensitizing layer 506 and a secondlight-blocking layer 510 disposed below the transport layer 507,separated from said transport layer 507 by means of secondary insulatinglayer 513.

The way of contacting the photo-active element 502 and the referenceelement 503 is somewhat different from what it has been described abovefor the previous examples. A first contact 508 a and a second contact508 b are provided at different levels on a same side of the pixel(namely, on the right-hand side in FIG. 5) and are circuitally connectedto a first end of the photo-active element 502 and of the referenceelement 503 respectively.

On the opposite side of the pixel 501 (on the left-hand side in theFigure), a common output contact 509 is circuitally connected to asecond end of the photo-active element 502 and of the reference element503. The output contact 509 comprises a vertical portion that extendsfrom the transport layer 505 of the photo-active element to thetransport layer 507 of the reference element.

The geometry of the photo-active elements of the previous examples canbe defined via patterning of the transport layer, which allows eithermaximizing the light-collection area or tailoring specific aspect ratiosfor the optimization of different performance parameters (such as forinstance, but not limited to, noise, responsivity, and resistance).

FIGS. 6A and 6B and 7A and 7B represent two pixel configurations basedon the example already discussed in the context of FIGS. 3A and 3B inwhich the pixel additionally comprises back-gate contacts.

In the example of FIGS. 6A and 6B, the pixel 601 comprises back-gatecontact 600 disposed below the photo-active element 106, between theinsulating layer 307 and the substrate 102. In this case, the insulatinglayer 307 is as a primary insulating layer associated to thephoto-active element 106 which, together with the back-gate contact 600,allows finely controlling the conduction and photosensitivity of saidphoto-active element 106.

As shown in FIG. 6A, the pixel 601 is a four-terminal device having thefirst contact 308 a and the second contact 308 b adapted to becircuitally connected, respectively, to first and second biasingcircuits providing substantially symmetrical first and second biasingvoltages V_(DD), V_(SS); the output contact 309 adapted to becircuitally connected to a readout circuit to deliver the photo-signalV_(OUT) generated at the pixel; and the back-gate contact 600 to providea gating voltage V_(GATE) to the photo-active element 106.

FIGS. 7A and 7B show another example of a pixel comprising back-gatecontacts. The pixel 701 has a layout similar to that of pixel 601, butdiffers in that it comprises not only a back-gate contact 700 disposedbelow the photo-active element 106 (between the insulating layer 307 andthe substrate 102) but also an additional back-gate contact 702 disposedbelow the reference element 107. Said additional back-gate contact 702is arranged between the insulating layer 307 and the secondlight-blocking layer 206.

Now, the insulating layer 307 is, at the same time, a primary insulatinglayer associated to the photo-active element 106 but also a secondaryinsulating layer associated to the reference element 107. Although inthis particular example the primary and secondary insulating layers areembodied as a same insulating layer, in other examples they can bedifferent layers arranged at a same or different levels in the layoutstructure of the image sensor.

The resulting pixel 701 can be operated as a five-terminal device inwhich its first and second contacts 308 a, 308 b are adapted to becircuitally connected, respectively, to first and second biasingcircuits providing first and second biasing voltages V_(DD), V_(SS) andits output contact 309 is adapted to be circuitally connected to areadout circuit to deliver the photo-signal V_(OUT) generated at thepixel 701. Additionally, the back-gate contact 700 is configured toprovide a gating voltage V_(GATE1) to the photo-active element 106 tofine-tune, for example, its photosensitivity, while the back-gatecontact 702 is adapted to provide a gating voltage V_(GATE2) to thereference element 107 to adjust its conductance.

Although in these examples the pixels 601, 701 are provided withback-gate contacts only, in other examples they may comprise,additionally or alternatively, top-gate contacts.

Referring now to FIG. 8Aa, it is there shown, in a bottom plan view, theblock diagram of an image sensor of the present invention. The imagesensor 800 comprises a plurality of pixels 801 arranged as atwo-dimensional array comprising a plurality of rows, each comprisingthe same number of pixels, aligned defining a plurality of columns. Theplurality of pixels 801 are arranged on a first area 802 of a substrate(not depicted in the Figure).

The image sensor 800 comprises a control unit operatively connected tothe plurality of pixels 801, which includes a first biasing circuit 803a for providing a first biasing voltage V_(DD), a second biasing circuit804 b for providing a second biasing voltage V_(SS), and a readoutcircuit 804. In particular, the second biasing voltage V_(SS) issubstantially symmetrical to the first biasing voltage V_(DD).

The first biasing circuit 803 a and the second biasing circuit 803 bcomprise, respectively, first row-select switches 805 a and secondrow-select switches 805 b to selectively bias the rows of the array.

The first and second row-select switches 805 a, 805 b make it possibleto sequentially enable only one row of the array at a time while leavingthe other rows disabled, which allows to daisy-chain the pixels 801 ofeach column of the array to the readout circuit 804, as it can beobserved in FIG. 8A. This greatly simplifies the interconnection of thepixels 801 to the readout circuit 804 and reduces the power consumptionof the image sensor 800 during operation.

Each pixel 801 comprises a photo-active element 809 circuitallyconnected between a first contact 811 a and an output contact 812, and areference element 810 circuitally connected between the output contact812 and a second contact 811 b. The structure of the pixels 801 is thesame as the one for the pixels 101, which has already been described indetail above in the context of the image sensor 100 in FIG. 1.

The image sensor 800 further comprises first conductive traces 815 a andsecond conductive traces 815 b that connect the first biasing circuit803 a and the second biasing circuit 803 b with, respectively, the firstcontact 811 a and the second contact 811 b of the pixels.

The readout circuit 804 includes a multiplexer 806 (depicted as aplurality of switches) that comprises as many input terminals 813 asthere are pixels 801 in each row and an output terminal 814. Each inputterminal 813 is circuitally connected to the output contact 812 of apixel of each row (in particular the pixels forming a column) by meansof third conductive traces 815 c provided in the image sensor 800.

The readout circuit further comprises an amplifier 807 operativelyconnected in series to the output terminal 814 of the multiplexer, and astorage element 808 operatively connected in series to the amplifier 807and configured to store a voltage proportional to the photo-signalgenerated in a pixel 801 of the plurality of pixels.

Upon readout, the control unit activates only one first row-selectswitch 805 a and only one second row-select switch 805 b at a time,biasing with balanced voltages only one row of pixels 801 of the array,while the pixels in the other rows remain disabled.

In this manner, only the pixels 801 in the selected row load the inputterminals 813 of the multiplexer 806. This makes it possible for a pixel801 of the selected row to be connected to the corresponding inputterminal 813 of the multiplexer 804 by means of the output contacts 812of the other pixels arranged in the same column as said pixel, and thethird conductive traces 815 c connecting said output contacts 812. Then,the photo-signal generated in each pixel 801 of the selected row canreach the readout circuit 804 without being disturbed by the pixels inthe other rows.

FIG. 8B shows another example of an image sensor that is similar intopology to the one just described in the context of FIG. 8A but with analternative readout circuit design. The image sensor 850 comprises aplurality of pixels 851 arranged on a first area 852 of a substrate andoperatively connected to a control unit that includes a first and asecond biasing circuits 853 a, 853 b circuitally connected,respectively, to first and second contacts 861 a, 861 b of each pixel,and a readout circuit 854 circuitally connected to an output contact 862of each pixel. The structure of the pixels, and that of the first andsecond biasing circuits of the image sensor 850 are similar to the onescomprised in the image sensor 800 and already described above.

The readout circuit 854 comprises as many amplifiers 857 as there arepixels 851 in each row, that is, the readout circuit 854 comprise anamplifier 857 for each column. Each amplifier 857 has an input terminal863, circuitally connected to the output contact of a pixel 851 of eachrow, and an output terminal 864. In addition, the readout circuit 854also comprises a storage element 858 that is connected in series to theoutput terminal 864 of each amplifier and configured to store a voltageproportional to the photo-signal generated in the pixels.

Additionally, the control unit of the image sensor 850 includes aninterconnection circuit 866 (a multiplexer in the example of FIG. 8B),operatively connected to the readout circuit 854 and that comprises anoutput node 867. The interconnection circuit 866 allows to circuitallyconnect, through the readout circuit 854, the output contact 862 of anyof the pixels of the array with the output node 867.

The image sensor 100 with non-local readout circuit described above inthe context of FIGS. 1, 2A, and 2B can be manufactured by means of amethod that comprises the steps of:

-   -   a) providing a transport layer 202 including at least one layer        of a two-dimensional material, and a photosensitizing layer 201        associated to a transport layer 201, on a first area 102 a of a        substrate 102;    -   b) providing a first biasing circuit 103 a, a second biasing        circuit 103 b and a readout circuit 104 in the control unit, the        first biasing circuit 103 a providing a first biasing voltage        V_(DD), the second biasing circuit 103 b providing a second        biasing voltage V_(SS) substantially symmetrical to the first        biasing voltage, and the readout circuit 104 being adapted to        read out the photo-signal generated by the light impinging on        the pixels 101;    -   c) arranging first selection means 105 a and second selection        means 105 b provided, respectively, in the first biasing circuit        103 a and the second biasing circuit 103 b outside the first        area 102 a of the substrate, the first selection means 105 a and        second selection means 105 b being adapted to selectively bias        one or more pixels 101 of said plurality that are to be read out        at a given time;        For each pixel 101 of the plurality of pixels, the method        further comprises:    -   d) defining a photo-active element 106 at a selected location of        the transport layer 202 and the photosensitizing layer 201        arranged on the first area 102 a of the substrate, and        circuitally connecting the photo-active element 106 between a        first contact 108 a and an output contact 109 provided in said        pixel 101;    -   e) arranging a non-photo-active reference element 107 proximate        to the photo-active active element 106 of said pixel, the        reference element 107 having a dark conductance that        substantially matches the dark conductance of the photo-active        element 106, and circuitally connecting the reference element        107 between said output contact 109 and a second contact 108 b        provided in said pixel 101;    -   f) circuitally connecting the first contact 108 a, the second        contact 108 b, and the output contact 109 of said pixel 101 to,        respectively, the first biasing circuit 103 a, the second        biasing circuit 103 b, and the readout circuit 104 of the        control unit.

FIGS. 10A-10G present the different steps involved in the process offabrication of the pixel 101 shown in FIGS. 2A and 2B.

In first place, as it can be seen in FIG. 10A, the second light blockinglayer 206 is selectively deposited, for example by means of a photomaskas used in conventional photolithographic process, on top of thesubstrate 102 only in the area that will be occupied by the referenceelement 107 of the pixel 101. Next (FIG. 10B), a passivation layercomprising an oxide is uniformly grown over the substrate to obtain theinsulating layer 207, which covers the second light-blocking layer 206and prepares the substrate for the deposition of the contacts of thepixel 101.

At this stage, the first contact 108 and the second contact 108 b aredefined at opposite ends of the pixel 101, together with the firstconductive trace 110 a and the second conductive trace 110 b (not shownin FIG. 10C) to provide the first and second biasing voltages V_(DD),V_(SS). Before defining the output contact 109 (illustrated in FIG. 10D)connected through its corresponding third trace 110 c to the readoutcircuit 104, it is necessary to grow an intervening insulating layer(such as the one described with reference to FIG. 9) to avoid theelectrical contact where the conductive traces 110 a, 110 b, 110 ccross.

Afterwards, one or more layers of a two-dimensional material areprogressively deposited on the substrate. Then, the transport layer 202of the photo-active element 106 and the transport layer 204 of thereference element 107 are etched, one next to the other, between thecontacts 108 a, 108 b, 109 previously defined (see FIG. 10E).

Next, FIG. 10F shows the deposition of a photosensitizing material ontop of the one or more layers of two-dimensional material, on which thephotosensitizing layer 201 of the photo-active element 106 and thephotosensitizing layer 203 of the reference element 107 are patternedabove their corresponding transport layer 202, 204.

Finally, the first light-blocking layer 205 is laid out selectively ontop of the photosensitizing layer 203 of the reference element, asdepicted in FIG. 10G. Optionally, at this final stage, a protectiveencapsulation layer made of a wide-bandgap dielectric material can bedisposed above the pixel 101.

The process of fabrication of the pixel shown in FIGS. 3A and 3B wouldbe essentially similar to the one just discussed, with the onlydifference that the deposition of the one or more layers of thetwo-dimensional material, and the subsequent etching of the transportlayers 202, 204, would be carried out prior to the definition of thecontacts 108 a, 108 b, 109.

Referring now to FIG. 11, it is there shown an example of an imagesensor capable of multispectral response. The image sensor 1100comprises a plurality of pixels arranged as a two-dimensional array andgrouped into clusters s1-s9. Each cluster comprises at least one pixelhaving a photo-active element with a photosensitizing layer sensitive toa different range of the spectrum. In this particular example, thephotosensitizing layer of the photo-active elements comprises quantumdots, whose size is progressively varied to tune their light absorptionproperties to different wavelengths.

Referring to FIGS. 13A and 13B, they show a further embodiment of theimage sensor of the present invention for which a light-concentratingstructure 1300 is arranged on top of the image sensor (above each pixelor above some of its pixels), specifically on top of an insulating layer1301 disposed above the photosensitive element (for a non-illustratedembodiment, it could be arranged directly on top of the image sensor,without an insulated layer in between), in order to enhance the responseof the photosensitive element. For the illustrated embodiment, thelight-concentrating element 1300 is a plasmonic bull's eye metallicstructure, although alternatively other geometries of plasmonic and/ordielectric structures that may consist of metals, dielectrics, heavilydoped semiconductors or graphene can be used, the choice of which isdetermined by the spectral range intended to be covered by the imagesensor.

For the embodiment of FIGS. 14A and 14B, the response of thephotosensitive element is further enhanced by adding a so-calledmicrolens 1400 on top of each pixel (only one pixel is shown in theFigure).

To demonstrate the spectral tunability of the photosensitive elements ofthe image sensor of the present invention, a prototype has been builtincluding an arrangement comprising several pixels differing betweenthem in that they are configured for being sensitive to different rangesof the light spectrum, in this case by means of the selection of thequantum dots (specifically the sizes thereof) which form theirrespective photosensitizing layers, one of which is configured for beingsensitive to Short-wave Infrared light (SWIR), another for Near Infraredlight (NIR) and another for visible light (VIS). The resulting waves aredepicted in FIG. 15, identified as SWIR-QDs, NIR-QDs and VIS-QDs.

In the plot of FIG. 16 data from a pixelated detector built according tothe image sensor of the present invention and that is illuminated withdiffracted light is shown, said data showing how the present inventionallows measuring the spectral decomposition of the impinging light.

In the plot of FIG. 17 data extracted from a graphene 4-pixelphotodetector linear array on a flexible and transparent substrate isshown. The sensors of the array have a 1×1 mm² dimension and a pixelpitch of 1.3 mm. The data is obtained by performing a reflectivephotoplethysmogram measurement on the finger of a person using a green(532 nm) light emitting diode as the light source. Each of the fourdepicted curves correspond to a different colour.

FIG. 12 represents the block diagram of an optoelectronic device, inparticular a wireless wearable device, which incorporates aphotodetector array according to the present invention.

The optoelectronic device 1200 comprises the image sensor 100 describedin FIG. 1 arranged on a flexible and/or stretchable substrate 1201,together with an analog-to-digital converter 1202, a control module 1203and a power supply module 1204 operatively connected to the control unitof the image sensor 100.

The control module 1203 is configured to provide control signals 1205 tothe control unit of the image sensor 100 to selectively bias and readout the pixels 101, and to receive a plurality of detected values 1206corresponding to the photo-signals read out from the plurality of pixels101 by the readout circuit 104. The analog-to-digital converter 1202 iscircuitally connected between the image sensor 100 and the controlmodule 1203 and is adapted to digitize the detected values 1206 beforethey are delivered to the digital circuitry embedded in the controlmodule 1203.

The power supply module 1204 is configured to provide the first andsecond biasing voltages V_(DD), V_(SS) to the first and second biasingcircuits 103 a, 103 b and to energize the active devices of the readoutcircuit 104.

The optoelectronic device 1200 further comprises an antenna 1207operatively interfaced with an RF-circuit included in the control module1203, and that allows the optoelectronic device 1200 to communicate viaa wireless connectivity standard (such as WiFi, Bluetooth, or ZigBee)with a user terminal 1208 provided with an antenna 1209, such as amobile telephone. The wireless link between the optoelectronic device1200 and the user terminal 1208 is advantageously used to program theoptoelectronic device 1200 remotely from the user terminal 1208, and totransfer data (such as for instance raw and/or processed data relatingto the detected values 1206 corresponding to the photo-signals read outfrom the pixels 101).

While the invention has been described with respect to some specificexamples, including presently preferred modes of carrying out theinvention, those skilled in the art will appreciate that there arenumerous variations and permutations of the above described image sensorand optoelectronic device using said image sensor, includingsubstitution of specific elements by others technically equivalent,without departing from the scope of the invention as set forth in theappended claims.

What is claimed is:
 1. An image sensor with non-local readout circuit,comprising a substrate, a plurality of pixels arranged on a first areaof the substrate, and a control unit operatively connected to theplurality of pixels and adapted to selectively bias said plurality ofpixels and read them out, wherein the control unit comprises: a firstbiasing circuit for providing a first biasing voltage; a second biasingcircuit for providing a second biasing voltage, the second biasingvoltage being substantially symmetrical to the first biasing voltagewith respect to a voltage reference; and a non-local readout circuit forreading out a photo-signal generated by light impinging on the pluralityof pixels; wherein the first biasing circuit and the second biasingcircuit comprise, respectively, first selection means and secondselection means to selectively bias one or more pixels of said pluralityof pixels that are to be read out at a given time, the first selectionmeans and the second selection means being arranged outside the firstarea of the substrate; and wherein each pixel of the plurality of pixelscomprises: a photo-active element comprising a photosensitizing layerassociated to a transport layer, the transport layer including at leastone layer of a two-dimensional material; a non-photo-active referenceelement disposed proximate to the photo-active active element, thenon-photo-active reference element having a dark conductance thatsubstantially matches a dark conductance of the photo-active element; afirst contact circuitally connected to the first biasing circuit; asecond contact circuitally connected to the second biasing circuit; andan output contact circuitally connected to the non-local readoutcircuit; wherein the photo-active element is circuitally connectedbetween the first contact and the output contact, and thenon-photoactive reference element is circuitally connected between theoutput contact and the second contact.
 2. The image sensor of claim 1,wherein the non-local readout circuit is arranged outside the first areaof the substrate.
 3. The image sensor of claim 2, wherein all pixels ofsaid plurality of pixels are absent of embedded readout electronics. 4.The image sensor of claim 1, wherein said substantial symmetrical firstand second biasing voltages have opposite signs with respect saidvoltage reference and a magnitude of one differs from a magnitude of theother in less than a 25%.
 5. The image sensor of claim 1, wherein thefirst biasing circuit and the second biasing circuit are independentbiasing circuits having their own independent control electronicsproviding the first biasing voltage and the second biasing voltage,respectively.
 6. The image sensor of claim 1, wherein thenon-photo-active reference element of at least one pixel of theplurality of pixels comprises a transport layer, said transport layer ofthe non-photo-active reference element including at least one layer of atwo-dimensional material.
 7. The image sensor of claim 6, wherein thenon-photo-active reference element of said at least one pixel furthercomprises a photosensitizing layer associated to the transport layer ofthe non-photo-active reference element.
 8. The image sensor of claim 7,wherein the non-photo-active reference element of said at least onepixel further comprises a first light-blocking layer disposed above thephotosensitizing layer and the transport layer of said non-photo-activereference element.
 9. The image sensor of claim 8, wherein thenon-photo-active reference element of said at least one pixel furthercomprises a second light-blocking layer disposed below thephotosensitizing layer and the transport layer of said non-photo-activereference element.
 10. The image sensor of claim 6, wherein, for said atleast one pixel, the transport layer of the non-photo-active referenceelement has a smaller area than the transport layer of the photo-activeelement.
 11. The image sensor according to claim 10, wherein, for saidat least one pixel, the transport layer of the non-photo-activereference element has the same shape as a transport layer of thephoto-active element.
 12. The image sensor of claim 1, wherein thenon-photo-active reference element of at least one pixel of theplurality of pixels is arranged between the substrate and thephoto-active element of said at least one pixel.
 13. The image sensor ofclaim 1, further comprising one or more primary insulating layersassociated to the photo-active element of the plurality of pixels. 14.The image sensor of claim 13, wherein at least one pixel of theplurality of pixels comprises: a back-gate contact disposed between thesubstrate and the photo-active element of said at least one pixel,between a primary insulating layer and the substrate, wherein saidprimary insulating layer is disposed between said photo-active elementand the substrate; and/or a top-gate contact disposed above thephoto-active element of said at least one pixel.
 15. The image sensoraccording to claim 6, further comprising one or more secondaryinsulating layers associated to the non-photo-active reference elementof the plurality of pixels.
 16. The image sensor of claim 15, wherein atleast one pixel of the plurality of pixels comprises: a back-gatecontact disposed between the substrate and the non-photo-activereference element of said at least one pixel, between a secondaryinsulating layer and the substrate, wherein said secondary insulatinglayer is disposed between said non-photo-active reference element andthe substrate; and/or a top-gate contact disposed above thenon-photo-active reference element of said at least one pixel.
 17. Theimage sensor of claim 1, wherein the plurality of pixels are groupedinto clusters, each cluster comprising one or more pixels; and whereinthe photosensitizing layer of the photo-active element of the one ormore pixels of each cluster is sensitive to a different range of lightspectrum.
 18. The image sensor of claim 1, wherein the plurality ofpixels are arranged as a two-dimensional array comprising a plurality ofrows, each row comprising the same number of pixels; and wherein thefirst selection means and the second selection means comprise,respectively, first row-select switches and second row-select switchesto selectively bias the plurality of rows of the array.
 19. The imagesensor of claim 18, wherein the control unit is operatively connected tothe first row-select switches and the second row-select switches, and isconfigured to sequentially read out the plurality of rows by activatingthe first row-select switch and the second row-select switch of one rowat a time.
 20. The image sensor of claim 18, wherein the non-localreadout circuit comprises: a multiplexer comprising as many inputterminals as there are pixels in each row and an output terminal, eachinput terminal of the multiplexer being circuitally connected to theoutput contact of a pixel of each row; and an amplifier operativelyconnected in series to the output terminal of the multiplexer.
 21. Theimage sensor of claim 20, wherein the non-local readout circuitcomprises a storage element configured to store a voltage proportionalto the photo-signal generated in a pixel of the plurality of pixels, thestorage element being operatively connected in series to the amplifier.22. The image sensor of claim 18, wherein the non-local readout circuitcomprises as many amplifiers as there are pixels in each row, eachamplifier having an input terminal, circuitally connected to the outputcontact of a pixel of each row, and an output terminal.
 23. The imagesensor of claim 22, wherein the non-local readout circuit furthercomprises a storage element connected in series to the output terminalof each amplifier, each storage element being configured to store avoltage proportional to the photo-signal generated in a pixel of theplurality of pixels.
 24. The image sensor of claim 1, wherein thesubstrate is made of a material which is at least one of: flexible,stretchable and transparent.
 25. The image sensor of claim 24, furthercomprising conductive traces that connect the first biasing circuit, thesecond biasing circuit and the non-local readout circuit with,respectively, the first, second and output contacts of the pixels of theplurality of pixels; wherein said conductive traces are made of aconductive material which is at least one of: flexible, stretchable andtransparent.
 26. An optoelectronic device comprising an image sensorwith non-local readout circuit, comprising a substrate, a plurality ofpixels arranged on a first area of the substrate, and a control unitoperatively connected to the plurality of pixels and adapted toselectively bias said plurality of pixels and read them out, wherein thecontrol unit comprises: a first biasing circuit for providing a firstbiasing voltage; a second biasing circuit for providing a second biasingvoltage, the second biasing voltage being substantially symmetrical tothe first biasing voltage with respect to a voltage reference; and anon-local readout circuit for reading out a photo-signal generated bylight impinging on the plurality of pixels; wherein the first biasingcircuit and the second biasing circuit comprise, respectively, firstselection means and second selection means to selectively bias one ormore pixels of said plurality of pixels that are to be read out at agiven time, the first selection means and the second selection meansbeing arranged outside the first area of the substrate; and wherein eachpixel of the plurality of pixels comprises: a photo-active elementcomprising a photosensitizing layer associated to a transport layer, thetransport layer including at least one layer of a two-dimensionalmaterial; a non-photo-active reference element disposed proximate to thephoto-active active element, the non-photo-active reference elementhaving a dark conductance that substantially matches a dark conductanceof the photo-active element; a first contact circuitally connected tothe first biasing circuit; a second contact circuitally connected to thesecond biasing circuit; and an output contact circuitally connected tothe non-local readout circuit; wherein the photo-active element iscircuitally connected between the first contact and the output contact,and the non-photoactive reference element is circuitally connectedbetween the output contact and the second contact, wherein theoptoelectronic device is a wearable device, and/or wherein theoptoelectronic device comprises a transparent panel on which the imagesensor is disposed.